1. Field of the invention
The present invention relates to a lithium secondary battery and, more specifically to improvement of negative electrode material used therefor to achieve better cycle characteristics and storage capability.
2. Description of the prior art
In recent years, lithium secondary batteries utilizing nonaqueous electrolytes have become spotlighted because of possible high-voltage design without taking decomposition voltage of water into consideration, thus differing from water-based secondary batteries utilizing aqueous electrolyte solutions, such as nickel-cadmium batteries.
Metallic lithium has been used as the negative electrode material of these lithium secondary batteries. In recent years, however, it has been pointed out that metallic lithium causes bad cycle characteristics due to growth of dendric deposits of lithium. Carbon materials, that occlude and discharge lithium ion solely during charge and discharge and have no problem of the above, are now studied as replacement of metallic lithium. Among these carbon materials, natural graphite is, having particularly high crystallinity and a large discharge capacity of 370 mAh/g, one of the most promising negative electrode materials.
However, natural graphite, being a natural product, contains in the crystals thereof impurities such as bound water and active terminal groups. These impurities react, during charge or discharge or during storage, with the electrolyte solution used to decompose and degrade it, whereby lithium secondary batteries with a negative electrode of natural graphite have had the problem of poor cycle characteristics and storage characteristics.
Accordingly, an object of the present invention is to provide a lithium secondary battery with a negative electrode of natural graphite having excellent cycle characteristics and storage characteristics.
The present invention provides a lithium secondary battery (hereinafter referred to as xe2x80x9cthe battery of the present inventionxe2x80x9d) comprising a natural graphite as a negative electrode capable of occluding and discharging lithium ion, said natural graphite having been heat treated at a temperature of at least 1800xc2x0 C.